This invention relates to a torque detector in which torque is detected by torsion produced in a shaft, which in turn produces a change in some observable quantity such as magnetic induction, electromotive force or phase angle.
An example of a conventional torque detector is shown in FIG. 2.42 on page 25 of the Japanese Industrial Electronics Measurements Handbook (published by Nikkan Kogyo Shimbun Sha, March 1970). This torque detector has a pair of magnetic excitation coils mounted on the outer circumference of a transmission shaft. The coils are spaced from each other by a predetermined distance. The torque detector also includes a pair of output pickup coils which are separated from each other by a predetermined distance in the axial direction of the transmission shaft, which direction is perpendicular to the direction of a line through the two excitation coils. Using the fact that the magnetic permeability of the transmission shaft varies corresponding to the amount of torsion in the transmission shaft produced by the torque, the torque is detected based on the voltage corresponding to the amount of torsion output by the said pair of pickup coils.
An example of such a torque detector is a steering torque detector installed in an automobile for use in conjunction with a steering device. In this case the transmission shaft is divided into two shafts in the axial direction and a magnetostrictive cylindrical body is affixed to ends of the first and second shafts. The shafts are coupled with each other through a stopper mechanism which couples them solidly together in the direction of rotation after their relative rotation exceeds a specified angle. In a low torque region until the stopper mechanism acts, all of the torque is transmitted through the magnetostrictive cylindrical body. In a high torque region, after the stopper mechanism engages, the torque is transmitted through the magnetostrictive cylindrical body and the stopper mechanism.
Because the relative rotation between the two shafts corresponding to the amount of torque is very small, the accuracy of the stopper mechanism directly affects the accuracy of the steering torque detection. Consequently, a problem is presented that in order to improve the accuracy of the torque detection, it is necessary to strictly control the accuracies of machining and assembly of parts constituting the stopper mechanism.